All of the sides of a cube are equal, then, the volume is given by the cube of the length of any side.
We need to find the biggest cubic value smaller than 80.
[tex]\begin{gathered} 3\times3\times3=27 \\ 4\times4\times4=64 \\ 5\times5\times5=125 \end{gathered}[/tex]
The largest cube has volume 64 cubic units, and the sides are 4 units long.
